1. Field of the Invention
The present invention relates to the field of rotary printing presses, and more particularly to variable cutoff printing in rotary printing presses.
2. Description of the Prior Art
Lithographic offset printing is typically done on a continuous process machine in which a web of material, or printing substrate, is fed through various stages of processing in a sequential order to result in the finished, printed product. The web is typically taken from an infeed roll at the beginning of the process and then passes through, for example, several print units, a dryer, a chill unit, a former section, a folder section, an angle bar section, a slitter and a cutting blade. The web is printed upon, cut, folded, and stacked en route to its final product stage.
The printing unit of a lithographic offset press typically contains an inking unit or ink rollers which supply ink to a printing plate, which printing plate is mounted on a plate cylinder. The printing plate contains the image to be printed. The ink is transferred from the printing plate to a printing blanket (i.e., the "plate-to-blanket" transfer), which printing blanket is mounted on a blanket cylinder. The printing blanket in turn transfers the ink onto the web of material (i.e., the "blanket-to-web" transfer). The plate and blanket cylinders have substantially parallel axes and are typically driven such that the surface speed of the plate coincides with the surface speed of the blanket to the degree necessary to achieve acceptable transfer of ink from the plate to the blanket.
Typically, the ratio of the diameters of the plate cylinder (i.e., the plate cylinder with the printing plate mounted upon it) and the blanket cylinder (i.e., blanket cylinder with the the printing blanket mounted upon it) is an integer number, providing such ratios as 1:1, 1:2, etc. One reason that an integer relationship between the plate and blanket cylinder diameters is used is so that the image from the plate is transferred to the same location on the blanket on successive rotations of the cylinders.
Latent images on the blanket cylinder are inked images that remain on the printing blanket after the blanket-to-web transfer due to incomplete transfer of a portion of the ink. Latent images create a need to have the same area on the printing blanket contact the same area on the printing plate on successive rotations. Thus, the plate transfers ink to the same area on the blanket on each revolution. In that manner, the adverse effects of latent images are minimized. However, if the diameters of the plate and blanket cylinders are not matched by a ratio of integers, then the printing plate will transfer an inked image onto a different area on the printing blanket on successive revolutions. Thus, the blanket may transfer to the web of material the primary image (i.e., the image from the most recent plate-to-blanket transfer) as well as a latent image (i.e., an image from a previous plate-to-blanket transfer which was not fully transferred to the web during the prior blanket-to-web transfer).
Because of the problem of doubling or printing a primary image on one location of the web and a latent image at a different location, it has historically been necessary to match the ratios of diameters of the plate cylinder and the blanket cylinder by an integer relationship so that latent images do not show up on the web of material. As a result, variable cutoff printing, or printing in which the image length, and hence the plate diameter, is changed from one print job to another is complicated by the need to match the blanket cylinder diameter to the plate cylinder diameter by an integer relationship.
The need to match the plate and blanket cylinder tangential and angular speeds in order to achieve acceptable print quality is well known in the art of offset printing. Further, a perfect match of the rotational position of the plate cylinder and blanket cylinder is required to insure that any latent image left on the blanket will be exactly refreshed by the plate image on subsequent revolutions.